To understand the impacts of dietary Glycyrrhiza uralensis on the immune responses of Lates calcarifer, the expression of immune-related genes including crp, c-3, c-4, mtor, mlst-8, eif4e, hsp-70, hsp-90, il-8il-8, il-10, tgfβ1, tnf, ifn-γ1, and mxf in L. calcarifer juveniles was evaluated in this study. Fish were fed experimental diets with G. uralensis levels of 0%, 1%, 3%, and 5% for 56 days. The results showed that dietary G. uralensis could improve the growth and survival of L. calcarifer and regulate the immune-related genes’ expression in L. calcarifer. Dietary G. uralensis significantly upregulated the expression level of crp, mtor, hsp-90, c-3, and c-4 genes in the liver of L. calcarifer, while hsp-70 gene expression was nearly downregulated. It did not upregulate the expression of elf4e and mlst-8 in the 1% and 3% inclusion groups, but it was the exact opposite in the 5% inclusion group. G. uralensis significantly affected the expression of il-8, il-10, tnf, ifn-γ1, mxf, and tgfβ1 in the head kidney of L. calcarifer. G. uralensis upregulated the expression of tnf and tgfβ1 consistently, but ifn-γ1 was at a low expression level. The expression of il-8 and il-10 was upregulated in the 1% group, while it was downregulated in the 5% group. The results from the present study indicate that dietary G. uralensis appeared to improve the immune function of L. calcarifer, and the optimum inclusion level should be between 1–3%.

This study investigates the effect of a sudden change in salinity for 48 h on the digestive enzyme activity of juvenile yellowfin tuna. The treatment included a control salinity of 32‰ in natural seawater and an experimental salinity of 29‰. Acute stress experiments were carried out on 72 juvenile yellowfin tuna (646.52 ± 66.32 g) for 48 h to determine changes in digestive enzyme activity in different intestinal sections over time (0 h, 12 h, 24 h, 48 h). The activities of pepsin, trypsin, α-amylase, lipase, and chymotrypsin in the digestive organs (stomach, foregut, and pyloric ceca) of juvenile yellowfin tuna were measured. Pepsin and pancreatic protease in the experimental group were significantly lower than in the control group (p < 0.05). α-amylase showed a fluctuating trend of decreasing and then increasing, and its activity trend was pyloric ceca > foregut > stomach. The lipase activity of gastric tissues decreased at the beginning and then increased, reaching a minimum at 24 h (2.74 ± 1.99 U·g protein⁻¹). The change of lipase in the pyloric ceca and foregut was increasing and then decreasing. The lipase activity trend was pyloric ceca > foregut > stomach. The chymotrypsin showed a decreasing and increasing trend and then stabilized at 48 h with a pattern of pyloric ceca > foregut > stomach. Similarly, the gut villi morphology was not significantly altered in the acutely salinity-stressed compared to the non-salinity-stressed. This study suggests that salinity may change the digestive function of juvenile yellowfin tuna, thereby affecting fish feeding, growth, and development. On the contrary, yellowfin tuna is highly adapted to 29‰ salinity. However, excessive stress may negatively affect digestive enzyme activity and reduce fish digestibility. This study may provide a scientific basis for a coastal aquaculture water environment for yellowfin tuna farming, which may guide the development and cultivation of aquaculture.

Yellowfin seabream, Acanthopagrus latus, is one of the most important species in terms of stock enhancement in China. However, using metagenomic techniques to explore the feeding habits and stomach microbiome of yellowfin seabream is still rare. The objective of this work was to study the feeding habits and stomach microbiome of yellowfin sea bream from Daya Bay through metagenomic analysis of different weight classes (≤50, 50–100, and >100 g). Whole-metagenome shotgun sequencing and morphological observation were used to investigate the stomach contents. The dietary composition and the community composition of the stomach microbiome of A. latus were examined. In this study, 153 species were detected in the eukaryotic composition of the stomach contents of yellowfin sea bream. At the species level, Mytilus edulis was the only species identified by both metagenomic analysis and morphological observation. The proportion of fish and bivalves was over 98%, but the diet changed little with body size. Larimichthys crocea, Scophthalmus maximus, and Seriola dumerili were the most abundant species among all samples. In total, 285 species were identified in the stomach microbiome of yellowfin sea bream. Bacterium 2013Ark19i, bacterium 2013Arg42i and Acinetobacter baumannii, first reported in the stomach contents of yellowfin sea bream, were the most abundant species of the stomach microbiomes. There was no difference in the biodiversity of the stomach microbiomes among the different body sizes. Overall, the composition of the yellowfin sea bream diet mainly consists of fish and bivalves. The use of metagenomics techniques is a promising approach for assessing the feeding habits of yellowfin sea bream. The results derived from this study can provide important information for evaluating the feeding ecology of yellowfin sea bream in Daya Bay.